The present invention relates to a thermal transfer sheet for use in a thermal recording system and more particularly to a thermal transfer sheet which avoids abrasion of a thermal head, collection of sheet debris on a thermal head, and sticking.
A dye sublimation thermal transfer sheet comprising a substrate film bearing, on one side thereof, a dye layer containing a heat-transferable dye and a hot-melt thermal transfer sheet having a hot-melt ink layer containing a wax which can be melt-transferred upon heating have hitherto been used as thermal transfer sheets for use in thermal printers, facsimiles and the like.
For the above conventional thermal transfer sheets, paper, such as an about 10 to 20 .mu.m-thick capacitor paper or paraffin paper, or a plastic film, such as an about 3 to 10 .mu.m-thick polyester or cellophane, is used as a substrate film, a dye layer formed of a dispersion or a solution of a dye in a binder resin or a hot-melt ink layer using a wax with a black pigment, such as carbon black, milled thereinto is provided on the substrate film, and, if necessary, a heat-resistant slip layer is provided on the back side of the substrate film for improving the slip property.
Predetermined sites of the thermal transfer sheet are heated and pressed according to image information from the back side of the substrate film by means of a thermal head or the like to transfer a dye in a site corresponding to a print portion of the dye layer onto an image-receiving sheet, thereby conducting printing.
In the case of the conventional thermal transfer sheet, however, in the course of printing of several thousand sheets, the coating of the heat-resistant slip layer or a filler contained in the coating is scraped off by the thermal head. Further, the coating of the heat-resistant slip layer is, in some cases, heat-fused to the thermal head to cause the collection of sheet debris on the head or the abrasion of a protective film of the thermal head. These unfavorable phenomena often lead to troubles such as uneven density.
In order to prevent the abrasion of a thermal head, EP-A 577051 proposes the use of soft particles, for example, particles having a Mohs hardness of not more than 3. When particles of natural minerals are used, however, the hardness often varies depending upon the kind and content of impurities even though the natural minerals are identical to each other with respect to the main component. Specifically, even though the main component has a Mohs hardness of not more than 3, the presence of a crystal having a Mohs hardness of not less than 3 as an impurity makes it impossible for particles of such a natural mineral to prevent the abrasion of a thermal head, suggesting that mere attention to the Mohs hardness alone never provides a solution to the problem of abrasion of the thermal head.
Further, when such soft particles are used, the particles are collapsed by heat or pressure applied during printing, causing the heat-resistant slip layer to be heat-fused to the thermal head. This often results in the collection of sheet debris on the thermal head or sticking.
If particles contained in the heat-resistant slip layer are protruded from the surface of the heat-resistant slip layer, the particles can scrape off the sheet debris, if any, to prevent the collection of the sheet debris on the thermal head. As described above, however, when soft particles which are less likely to abrade the thermal head are used, the particles are collapsed by heat or pressure applied during printing, making it impossible to scrape off the sheet debris on the head.